1. Field of the Invention
The present invention relates to a receiving apparatus and channel estimating apparatus that receive send signals subject to frequency hopping and perform channel estimation and, in particular, to a receiving apparatus and channel estimating apparatus that average multiple channel estimation results in a time domain and remove an influence of noise.
2. Description of the Related Art
A wireless network has received attentions as a system for freeing from wiring in a wired communication method in the past. The standard specifications relating to a wireless network may include IEEE (The Institute of Electrical and Electronics Engineers) 802.11.
OFDM (Orthogonal Frequency Division Multiplexing) transmission scheme has been expected as a technology that avoids the deterioration of transmission quality due to the fading of wireless signals and increases the speed and/or quality of the wireless transmission. IEEE 802.11a/g, for example, has adopted OFDM modulation scheme as standard specifications for a wireless Local Area Network (LAN). Though a wireless communication scheme called “Ultra Wide Band (UWB) Communication” employing a significantly wide frequency band has received attentions in recent years, the standardization conference in IEEE 802.15.3 has reviewed DSSS (Direct Sequence Spread Spectrum)-UWB scheme and OFDM_UWB adopting OFDM modulation scheme.
In OFDM transmission scheme, frequencies for carriers are defined such that the sub-carriers can be orthogonal to each other within a symbol period. The expression, “the sub-carriers can be orthogonal to each other”, refers to that the peak point of the spectrum of an arbitrary sub-carrier can typically agree with the zero point of the spectrums of other sub-carriers. In OFDM modulation scheme, since send data is divided into and is transmitted in multiple carriers at different frequencies, the band of each of the carriers is narrowed, which may significantly increase the efficiency of frequency application and the resistance to the interference of frequency-selective fading.
A small place such as an office has a problem of the coexistence of multiple wireless networks in a communication environment. Accordingly, frequency hopping (FH) scheme is adopted for flexibly changing the frequency band to use. In this communication scheme, packets are exchanged by changing the frequency randomly every time, and the influence from another system may disturb the communication. However, the continuous change of the frequency may hardly interrupt the communication. In other words, the coexistence with other systems is allowed with good resistance to fading and easy scalability.
For example, a multi-band scheme of OFDM_UWB scheme as described above has been reviewed in which each of the bands from 3.1 GHz to 10.6 GHz defined by the Federal Communications Commission (FCC) is divided into multiple sub-bands each having 528 MHz, and frequency hopping (FH) is performed among the sub-bands.
FIG. 7 shows an example of the frequency assignment defined by the multi-band OFDM_UWB communication scheme (which will be called “MB-OFDM” hereinafter) (refer to “MBOFDM PHY Specification Final Release 1.0,” Wimedia alliance, Apr. 27, 2005 (Non-Patent Document 1), for example). In the shown example, the band of 5 GHz to be used by a wireless LAN is a Null band, and the other band is divided into thirteen sub-bands. The sub-bands are divided into four groups of A to D, and communication is performed by managing the frequency of each of the groups.
FIG. 8 shows a state in which data transmission is performed in the multi-band OFDM scheme by performing frequency hopping on an OFDM symbol in a time domain. In the shown example, Group A having Bands #1 to #3 is used, and frequency hopping is performed by changing the center frequency for each one OFDM symbol, and OFDM modulation is performed by using inverse fast Fourier transform/fast Fourier transform (IFET/FET) having 128 points.
By the way, wireless communication has a problem that a signal transmitted from a transmitting apparatus is subject to the influence of a channel characteristic on the transmission path before being received by a receiving apparatus. More specifically, since a send signal has the phase rotated and the signal amplitude changed on the transmission path, the original data may not be properly decoded from the received signal. Thus, the receiving apparatus may need to estimate the channel characteristic and perform channel correction for correcting the amount of phase rotation and the amplitude of the received signal.
The channel estimation is generally achieved by exchanging a known sequence between a transmitting apparatus and a receiving apparatus. In other words, a transmitting apparatus transmits a training sequence for channel estimation (called “channel estimation sequence” hereinafter) including a preamble of the packet, and the receiving apparatus multiples the received preamble by the known sequence. Thus, the amount of phase rotation and the amount of amplitude change on the transmission path are obtained, and the channel correction can be performed that on the payload by performing an operation of reversing the phase by the amount of phase rotation and putting the amplitude back.
In OFDM transmission scheme, one channel estimation sequence includes one OFDM symbol, for example. Partial sub-carriers of a channel estimation sequence may be lost due to the influence of fading, and proper channel estimation may not be performed as a result. Therefore, moving averaging processing is generally performed on the adjacent multiple sub-carriers in a frequency domain of the sub-carriers included in the OFDM symbol for channel estimation, whereby the influence of fading is removed. Furthermore, in order to avoid the decrease in accuracy of the channel estimation due to thermal noise, which is added during radio frequency (RF) analog processing, a transmitting apparatus transmits a channel estimation sequence multiple times while a receiving apparatus averages the multiple channel estimation values in a time domain. Thus, the influence of thermal noise can be reduced, and the accuracy of the channel estimation can be improved (refer to JP-A-2000-358010, Paragraph No. 0013 (Patent Document 1), for example).
FIG. 9 schematically shows mechanisms for channel estimation and channel correction in OFDM transmission scheme. In this case, the preambles before a payload of a transmission packet from a transmitting apparatus include a synchronization sequence and a channel estimation sequence as shown in FIG. 10. The channel estimation sequence includes two known sequences each having one OFDM symbol.
The receiving apparatus performs RF processing on the received signals transmitted as time-axis signals, performs FFT (that is, OFDM demodulation) thereon and demultiplexes them into sub-carriers aligned in the frequency domain (not shown). Then, the channel estimation part of the packet is extracted and is multiplied by a known channel estimation sequence that the receiving apparatus holds, whereby the correlation value is obtained.
Some sub-carriers aligned in the frequency domain may be lost due to fading caused during transmission. The influence of fading is removed by averaging each sub-carrier included in the channel estimation sequence and adjacent multiple sub-carriers in the frequency domain. Furthermore, the accuracy of channel estimation is improved by averaging, in the time domain, the channel estimation values having multiple OFDM symbols for channel estimation. Then, the channel estimation result therefrom is multiplied by the payload part of the packet, whereby the operation for correcting the amplitude change and phase rotation due to a channel characteristic is performed.
The averaging, in the time domain, of the channel estimation results among multiple OFDM symbols can provide a channel estimation result with higher accuracy excluding the influence of thermal noise, which is added when RF analog processing is performed thereon.
However, a system applying frequency hopping has a problem that the averaging in the time domain causes a frequency error in channel estimation values of bands. Generally, a channel estimation sequence of each frequency channel is transmitted in a time division manner in accordance with a hopping pattern (that is, for each cycle for performing frequency hopping). For example, in a communication system that performs frequency hopping on three bands of Bands A, B and C, when a channel estimation sequence is transmitted in a time division manner by using two OFDM symbols for each band, such as A1 and A2, B1 and B2 and C1 and C2 as shown in FIG. 11, the interval between A1 and A2 increases by the time equivalent to the hopping interval. The phase of the received OFDM signal drifts on each of the bands during the time interval, which causes a large frequency error and an error in channel estimation values in averaging in the time domain. As a result, the bit error rate is deteriorated, which is a problem.